The Four Halachic Domains on Shabbos
The laws of carrying use four halachic domains. Their names describe legal status under hilchos Shabbos, not simply who owns or uses the place.
Reshus hayachid
A reshus hayachid is a private halachic domain with the required dimensions and enclosure. A home is the familiar example, but the term can also include a courtyard, fenced area, or other properly enclosed place used by many people.
The word hayachid describes its halachic status, not the number of people present. A crowded dining hall or enclosed residential complex may still be a reshus hayachid, while a space owned by one person may lack that status if it is not enclosed as halacha requires.
Reshus harabim
A reshus harabim is the Torah's public domain. Width, openness, public use, the form of the route, and according to many Rishonim the passage of shishim ribo are central to its definition.
A true reshus harabim cannot ordinarily be enclosed by the same simple arrangement used around a karmelis. Questions about sixteen amos, mefulash streets, shishim ribo, gates, and the city as a whole therefore come before the placement of poles and wires.
Karmelis
A karmelis is not a Torah-level reshus harabim, but Chazal prohibited carrying within it and between it and a reshus hayachid. Many unenclosed shared areas fall into this category.
The rabbinic prohibition is fully binding. Describing a street or open area as a karmelis does not mean that carrying is permitted; it means that a valid enclosure and the other required arrangements may be able to permit it.
Makom petur
A makom petur is a limited exempt place defined by its dimensions and setting. It is not a general device for carrying through the street, and its use can involve additional restrictions.
Whether a post, narrow ledge, utility fixture, or similar place qualifies depends on its height, width, surrounding domain, and use. A person should not improvise a route through supposed exempt places as a substitute for a valid eruv.